Polymer nanocomposite materials have been the subject of research in recent years because of their potential for advanced properties and multi-functionality. Composites using graphene are of particular interest for a wide range of applications, as graphene combines outstanding mechanical, electrical, and barrier properties with a high surface area [1,2].
The fabrication of macroscopic amounts of single-layer graphene sheets many micrometers in size for large-scale application in nanocomposite materials is challenging. In order to study their electronic, mechanical, and other properties, individual graphene sheets have been produced by mechanical exfoliation at very low yields, but this approach is not well-suited for large-scale applications [6]. A more promising approach for high yields is exfoliation of graphite oxide: graphite is treated with strong acids and oxidizing agents to produce graphite oxide, in which the addition of oxygen-containing functional groups on the surfaces of sheets increases the lamellar spacing between sheets and reduces the van der Waals forces holding the sheets together. This material can be exfoliated either thermally at a very high temperature (e.g., 1050° C.) in a tube furnace in an inert gas [1] or by sonication in solvents [7]. The high demand in energy and time can create challenge in processing. The tube furnace exfoliation simultaneously exfoliates and reduces graphite oxide, thereby removing the vast majority of the functional groups from the oxidized material. These functional groups are released primarily as CO2 and H2O gas. The resulting sheets are chemically similar to graphene. Accordingly, they are—like carbon nanotubes—very hydrophobic and thus difficult to handle in liquid processing. As a result, stable dispersions can only be achieved in a small number of relatively exotic solvents.
This difficulty is avoided when graphite oxide is exfoliated acoustically in solvents, such that the sheets retain a significant amount of their functional surface groups. Consequently, stable dispersions can be achieved in a large range of solvents including water, alcohols, and dimethylformamide. Due to this broad compatibility with solvent processing, these functionalized graphene sheets (FGS) [7] have been used in composite materials instead of graphene due to its ease of preparation and its compatibility with many polymers. Unfortunately, these more highly functionalized graphene sheets do not have the advantageous mechanical and thermal properties or conductivity as does graphene [8,9]. Thus, it would be desirable to benefit from the unique properties of graphene without sacrificing the ease of preparation of functionalized graphene sheets. Methods of reducing graphene oxide while in aqueous dispersions have been developed [6]. These methods, however, need surfactants to be added in considerable amounts to avoid collapse of the dispersion [7], since the reduction renders the sheets hydrophobic. The presence of such surfactants is not desired in the production of nanocomposites, where the interface between the sheets and the polymer matrix is important to the performance.
Thus, a need exists to provide a better method of fabricating a graphene-containing composite from graphene oxide.